In recent years polyimides have had increasing use as thermosetting resins in high performance applications, as the matrix resin for reinforced composites in spacecraft and missiles and for syntactic foams, as well as for laminates in printed circuit boards and other electronic applications. When polyimide resins are cured they generally afford a polymer with a high glass transition temperature and excellent chemical (environmental) stability with particularly good resistance to moisture and to oxidative degradation at elevated temperatures. However, cured resins typically are extensively crosslinked leading to products which are very brittle, that is, having low fracture toughness.
Many bismaleimides manifest the unfortunate property of beginning to polymerize at a temperature which is at or just above the melting point of the monomer, that is, the temperature differential between melting and onset of polymerization is small. As a result it is difficult to maintain the uncured resin in a fluid state, and the accompanying difficulty in attaining a homogeneous melt leads to well documented processing difficulties. The patentee of U.S. Pat. No. 4,464,520 addressed this problem and provided a class of bismaleimides (BMIs) with increased pot life, therefore a "larger processing window". However, the compositions taught there still afforded cured polymers which were brittle.
Because the brittleness of the cured product arises from extensive crosslinking during polymerization, many efforts have been made to reduce the crosslink density in the cured product to afford toughened BMIs without adversely impinging on other desirable properties. See H. D. Stenzenberger et al., 19th International SAMPE Technical Conference, Oct. 13-15, 1987, pages 372-85. Among approaches to toughening BMIs, the authors include the reaction of o,o'-dialkyl bisphenol A with a BMI. However, they state that all dialkyl phenyl compounds react via an "ene" type reaction in which the alkyl moiety rather than the hydroxyl group reacts with the BMI.
One general approach has been to react BMI monomers at their carbon-carbon double bonds with certain reactive bifunctional reagents having active hydrogens to afford Michael addition products. This reaction and the resulting Michael adduct may be exemplified using a diamine as the reactive bifunctional reagent by the equation, ##STR1## As the foregoing equation shows, Michael addition reduces double bond density in the BMI monomer (or oligomer) resulting in a lower crosslink density in the cured product. The diamine also can be viewed as a chain extender in addition to its function of reducing crosslinking density.
Michael addition generally is a base-catalyzed reaction, and since amines as bases serve as their own catalysts this is one reason why amines usually are quite reactive in Michael addition. Where alcohols are used, the reaction with nitrogen-substituted maleimides requires a base catalyst as an additional component; A. Renner et al., Helv. Chim. Acta., 61, 1443 (1978). These workers also have given an instance of the reaction via Michael addition of a polyhydric phenol (bisphenol A) to a typical BMI monomer, with chain extension requiring a discrete base catalyst. However, the use of a third component as a catalyst along with a chain extender polyhydric phenol generally is undesirable since the resulting product retains the catalyst as a component which might significantly degrade the performance of the final cured resin. The necessity of using basic catalysts for chain extension with polyols is particularly unfortunate, since a significant advantage of polyols is that they are non-carcinogenic whereas aromatic diamines used as chain extenders often are carcinogenic. The reaction between polyols and the carbon-carbon double bonds of typical BMIs in the absence of catalysts has been believed to be insignificant as the prior art demonstrates.
Hitachi Chemical in published patent application J61145225-A shows the reaction of a mixture of a BMI, a polyol, a trialkylisocyaurate, and an acid catalyst. It is believed that the hydroxyl groups of the polyol are not reacting with carbon-carbon bond of the bismaleimide, but instead the polyol reacts with the trialkyl isocyanurate while the BMI undergoes homopolymerization. Rakoutz and Balme in Polymer Journal, Vol 19, No. 1 pp. 173-184 (1987) show a similar reaction.
Soviet Union published application SU1058976-A shows the reaction of an oligophenoldisulfide with a bismaleimide. The oligophenoldisulfides are stated to have a molecular weight of 300-1030 and to cure by breaking the disulfide groups followed by reaction of the radicals formed with bismaleimide. While the applicants suggest that hydroxyl groups could be connecting to the carbon-carbon double bond of the BMI, where a Novolac is used (Example 11) the flex strength is lower than with the BMI alone (Example 12) and much lower than those examples where a significant proportion of the oligophenoldisulfide is used. Thus, it is concluded that the reaction of the sulfur-containing radicals is responsible for the improved performance and the effect of reacting hydroxyl groups with the BMI is small. Thus, one skilled in the art could conclude that the reaction of Novolacs with conventional BMIs in the absence of a catalyst are not significant.
The bismaleimides of U.S. Pat. No. 4,464,520, representative of which is the structure ##STR2## could be expected to undergo Michael addition sluggishly, if at all, because of the relatively hindered nature of the maleimide double bond. Quite unexpectedly it was found that not only did such materials undergo Michael addition, but in fact they reacted facilely with the less reactive polyhydric phenols. But not only did the polyhydric phenols readily react with the aforementioned BMIs, they did so in an uncatalyzed reaction, that is, in the absence of a base catalyst. This totally unexpected behavior afforded cured resins containing no performance-degrading components and led us to examine some relevant performance characteristics of representative chain-extended cured resins. We have found that relative to the parent cured resin, chain extension generally has reduced brittleness and improved the toughness of the cured resin, with the latter having a superior dielectric constant and loss factors and comparable coefficients of thermal expansion and chemical resistance. Most surprisingly, the chain-extended BMIs have wider processing windows than either the parent or diamine chain-extended BMI resins.